Thread: Carb question

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Originally Posted by NTFDAY

Vacuum from the engine determines when the secondaries begin to tip in on a vacuum secondary carb to eliminate engine bog.

The bog you mention, what's the cause?

Originally Posted by ceh383

The bog you mention, what's the cause?

A carburetor works off Bernoulli's Principle. Airflow velocity is increased at the venturi(s) because they are smaller than the carb inlet at the airhorn. This increase of velocity creates a vacuum that pulls fuel from the reservoir bowl to mix with the incoming air. If there is not enough air flow (thus insufficient vacuum) to pull fuel and atomize it properly, then you will experience a bog. That's what happens when all four barrels open at low rpm's. Too much carburetor or too little motor, however you want to look at it.

Most carburetor manufacturers will recommend a vacuum secondary carb if you are using a converter that stalls at less than 3000 rpm's. Over 3000, there's enough airflow to support all four barrels and get a good mix, assuming you have sized the carburetor properly for the size of the motor. For max power street and strip, a good rule of thumb is cubic inches times 2 = CFM.

Originally Posted by techinspector1

A carburetor works off Bernoulli's Principle. Airflow velocity is increased at the venturi(s) because they are smaller than the carb inlet at the airhorn. This increase of velocity creates a vacuum that pulls fuel from the reservoir bowl to mix with the incoming air. If there is not enough air flow (thus insufficient vacuum) to pull fuel and atomize it properly, then you will experience a bog. That's what happens when all four barrels open at low rpm's. Too much carburetor or too little motor, however you want to look at it.

Most carburetor manufacturers will recommend a vacuum secondary carb if you are using a converter that stalls at less than 3000 rpm's. Over 3000, there's enough airflow to support all four barrels and get a good mix, assuming you have sized the carburetor properly for the size of the motor. For max power street and strip, a good rule of thumb is cubic inches times 2 = CFM.

I knew someone would come on that could explain it a lot better than I could. Thanks, Richard

Alright, good answer. I have seen Cfm = (engine size/2) x (maximum rpm/1,728)] / VE % will give the correct cfm. Using this formula I would need a 546cfm carb. Using CID x 2 I would need a 710cfm carb....Big difference. A debate on correct sizing is not what I'm looking for here. Here's what I'm after... I have a 390cfm double pumper that came off a circle track(short track) car with a 355cid engine that lived in the 5500 - 8500 rpm range. I'm wondering if it will be worth the trouble to put it on my motor. It has a 355cid built for 2000 - 6000 rpm range, but I have an automatic. The carb is one of THESE.

Originally Posted by ceh383

Alirght, good answer. I have seen Cfm = (engine size/2) x (maximum rpm/1,728)] / VE % will give the correct cfm. Using this formula I would need a 546cfm carb. Using CID x 2 I would need a 710cfm carb....Big difference. A debate on correct sizing is not what I'm looking for here. Here's what I'm after... I have a 390cfm double pumper that came off a circle track(short track) car with a 355cid engine that lived in the 5500 - 8500 rpm range. I'm wondering if it will be worth the trouble to put it on my motor. It has a 355cid built for 2000 - 6000 rpm range, but I have an automatic. The carb is one of THESE.

My own personal opinion is that it will be more trouble than it is worth.

By either of the two formulas given to determine carb size this would be an under sized carb. Given that, would there be sufficient vacuum to pull fuel and atomize it properly?

Yes it will work just as a Stromberg 97 would work if you could mount it, but in my opinion drive ability will be adversely affected.